A broadband communications system 100, such as a two-way cable television system, is depicted in FIG. 1. The communications system 100 includes headend equipment 105 for generating forward signals that are transmitted in the forward, or downstream, direction along a communication medium, such as a fiber optic cable 110. Coupled to the headend 105 are several hubs 115 that serve sites that may be miles away from the headend 105. Included within the hubs 115 is fiber equipment for further transmission of the optical signals to optical nodes 120 that then convert the optical signals to radio frequency (RF) signals. The RF signals are further transmitted along another communication medium, such as coaxial cable 125, and are amplified, as necessary, by one or more distribution amplifiers 130 positioned along the communication medium. Taps 135 included in the cable television system split off portions of the forward signals for provision to subscriber equipment 140, such as set top terminals, computers, and televisions.
In a two-way system, the subscriber equipment 140 can also generate reverse electrical signals that are transmitted upstream, amplified by any distribution amplifiers 130, converted to optical signals by the optical node 120, and provided to the headend equipment 105. More recently, however, new cable applications, such as interactive multimedia, Internet access, and telephony, are increasing the demand for additional reverse path capability. Cable operators are redesigning the networks 100 to increase the total reverse bandwidth and further refine the network to become two-way active. Some of the difficulties in the growth of the reverse path are that the conventional methods used to transmit reverse signals from a hub 115 to a headend 105 continue to become more complex and expensive as the numbers of reverse paths grow, i.e., more requirements for optical transmitters, optical receivers, and the connecting fiber links. Networks 100 are also beginning to increase the physical territory to include areas that may not have been previously serviced with cable television and considered to be “green space.”
To address the increased demands on the reverse path, the analog signals within the reverse frequency range, such as from 5 MHz to 42 MHz, can be converted to digital signals. A simplified digital reverse system that can be used in a typical cable television system is depicted in FIG. 2. Digitizing the reverse bandwidth as shown in FIG. 2 allows the operator to increase the reverse path capacity that is demanded by the growing interactive applications. Briefly, a plurality of digital transmitters 205 each including an analog-to-digital (A/D) converter 208 receives analog electrical signals from a number of connected subscribers and converts the analog signals to digital optical signals. Linked to each transmitter 205 is a digital receiver 210 that includes a digital-to-analog (D/A) converter 215 located further upstream in the network 200. The D/A converter 215 converts the received digitized optical signals back to analog electrical signals for delivery to the headend and further processing. The A/D and D/A converters typically operate around 100 Mega samples per second (Ms/s) with each sample comprising around 10 bits to 12 bits. Consequently, the resulting bit rate of the transmitters 205 and the receivers 210 are around 1 Giga bit per second (Gb/s). Also, the 1 Gb/s data stream is produced regardless of whether there is an RF signal present at the transmitter input or not. Additionally, each reverse link included in the network requires its own digital transmitter 205 and digital receiver 210. As subscribers upgrade their packages to include more advanced services, there may be more links required throughout the network 200 to handle the increased reverse traffic.
It will be appreciated that the digital transmitters 205 and the digital receivers 210 can be utilized in a number of broadband communications products and applications, such as digital reverse transmission from an optical node 120 to the headend 105 or from a hub 115 to the headend 105.
The significant number of transmitters, receivers, and connecting fiber presents an inefficient network design. Another major concern is the impact on the reverse path when operators begin pulling fiber closer to the subscriber. More specifically, the reverse, or upstream, path cannot optically combine the reverse signals coming from the digital transmitters 205. In other words, the links need to remain separate and cannot be combined. In contrast, in the forward, or downstream, path an optical splitter can be used to split the optical signal into a plurality of optical paths where each are then provided to a pocket of homes or, in the case of longer fiber runs, directly to a very limited number of homes. Since the signals cannot be combined in the reverse path, an increased number of reverse digital transmitters, digital receivers, and connecting fiber are required throughout the communications system in order to adequately transmit and receive the reverse RF signals from each subscriber. Thus, what is needed is a method and apparatus for combining the reverse RF signals in the optical domain in order to decrease the amount of required equipment and efficiently receive reverse RF signals at the headend.